CVE-2026-90898: Bifrost unauthenticated remote code execution via MCP stdio client registration

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 2.3% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-09-14 · updated 2026-09-18
Weakness: CWE-284

Description

Bifrost registers MCP clients through its management API. A stdio client is a command plus args. Bifrost starts that program in the gateway the moment the client is added. No MCP handshake required. The default is governance.auth_config.is_enabled=false. Auth off means every caller is a local admin. One unauthenticated POST /api/mcp/client is enough to run a program as the Bifrost process user (appuser on the official image). transports/v2.1.0 refuses an unauthenticated stdio registration with 403. transports/v2.0.0 still allows it.

Affected products

maximhq Bifrost: before 2.1.0 — fixed in 2.1.0

How to fix

Upgrade to a fixed version: maximhq Bifrost 2.1.0. Check the vendor advisory for mitigations.

References

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